- 01Blood potassium shows about 2% of the body's store
About 98% of the body's potassium sits inside cells, so a blood potassium reports only around 2% of it 3. This explains why insulin or an albuterol inhaler can drop the blood level without any loss, why a small fall from real losses (0.3 mmol/L) can mean about 100 mmol missing from the body 2, and why the cause, not the number alone, decides what a doctor does next.
Low potassium first shows up as tiredness, weak or cramping muscles, constipation and a heart that flutters or skips, because potassium sets the electrical charge that muscles and the heart run on. Most people feel nothing while potassium stays above about 3.0 mmol/L, and symptoms become likely below it 12. That is why many people only find out from a routine blood panel.
Where an example potassium of 3.2 fallsmmol/L
Each of these values has its own page: a potassium of 3.5, 3.4 and 3.2. There is a separate page for high potassium.
What to do with a mild low result
- 3.0 to 3.4 with no symptoms: ask your clinician about a repeat potassium, usually within days to a couple of weeks, with magnesium and bicarbonate on the same draw.
- Bring a list of everything you take, especially water pills (diuretics), laxatives, steroids, asthma inhalers and any supplements.
- If your blood pressure is high too, ask whether an aldosterone-to-renin test makes sense (see the section on high blood pressure below).
- Below 3.0, or a low result with weakness or cramps: arrange a prompt review within a few days. Below 2.5, or any low result with palpitations, chest pain, fainting, trouble breathing or weakness that makes it hard to stand: get care the same day.
The signs of low potassium, by body system
Potassium sets the electrical charge across every muscle and nerve cell. When blood potassium falls, those cells become harder to fire, so the symptoms of low potassium cluster in the muscles, the heart and the gut 3.
- Muscles: tiredness, weakness (often legs first), cramps, aches and twitching. See muscle cramps and fatigue. In severe low potassium, weakness can progress to paralysis, and muscle breakdown (rhabdomyolysis, where damaged muscle leaks into the blood) can follow 1.
- Heart: palpitations, skipped or extra beats, and in severe cases dangerous rhythms. See heart palpitations.
- Gut: constipation, bloating and, when severe, a stalled bowel (ileus) 2.
- Kidneys: thirst and passing more urine than usual, because prolonged low potassium blunts the kidney's ability to concentrate urine 2.
- Breathing: in severe low potassium, the breathing muscles can weaken and cause shortness of breath 1.
Low potassium can also show up on an electrocardiogram (ECG, the heart tracing) as tell-tale changes in the shape of each beat, which is why a doctor may order one when potassium is moderately or severely low, or there are symptoms 1. This matters most for people with heart disease or who take digoxin, a heart drug whose toxicity rises when potassium is low. For them, a US expert consensus advises keeping potassium at 4.0 mmol/L or higher 7; the exact target is set by their own doctor.
Read these beside your potassium
How low is too low
| Potassium (mmol/L) | Grade | Typical symptoms | What usually happens next |
|---|---|---|---|
| 3.0 to 3.4 | Mild | Often none; sometimes tiredness or cramps | Repeat test, look for the cause, review medicines |
| 2.5 to 2.9 | Moderate | Weakness, cramps, constipation, palpitations | Prompt review, ECG, magnesium check, prescribed replacement |
| Below 2.5 | Severe | Marked weakness, paralysis, arrhythmia | Emergency care, heart monitoring, IV potassium |
Is the low potassium result real?
A single mild low deserves a repeat before anyone acts on it. Two lab effects run in opposite directions. Haemolysis (red cells bursting in the tube) pushes potassium up, so a haemolysed sample can hide a true low, never cause one. A very high white cell count, or a sample left warm for hours before processing, can make potassium read falsely low, called pseudohypokalemia 1. Hard fist clenching during the draw, or blood taken from an arm with a drip running, can also distort the result.
Some lows are real but brief. Insulin, beta-2 agonist inhalers such as albuterol (salbutamol), and alkalosis (blood that is less acidic than normal) all push potassium into cells, so the blood level drops without any potassium leaving the body 3. A test taken during an asthma flare, or soon after an insulin dose in someone with type 2 diabetes, can read low for that reason. The blood test prep guide covers the steps that keep a sample clean.
What causes low potassium
Every low potassium fits one of three buckets: potassium is lost from the body, potassium shifts into cells, or potassium intake is very low. Losses are by far the most common, and thiazide and loop diuretics are the leading drug cause in people outside hospital 2. A central player is aldosterone, a hormone from the adrenal glands that makes the kidneys keep salt and lose potassium.
| Losing potassium | Shifting potassium into cells | Not taking enough in |
|---|---|---|
| Diuretics (thiazide, loop) | Insulin | Eating disorders |
| Vomiting, diarrhoea, laxative overuse | Albuterol and other beta-2 agonists | Heavy alcohol use |
| High aldosterone, Cushing's syndrome (too much cortisol), high-dose steroids | Alkalosis | Very restrictive diets |
| Low magnesium | Refeeding after starvation (eating again after a long fast pulls potassium into cells) | |
| Black licorice in large amounts; rare inherited kidney conditions (Gitelman, Bartter) | Thyroid-related periodic paralysis (rare attacks of sudden weakness with an overactive thyroid) |
Low intake usually causes a low only alongside another cause. Black licorice contains glycyrrhizin, which lets the body's own cortisol act like aldosterone on the kidney, so potassium is lost in the urine 1.
Magnesium deficiency is frequently associated with hypokalemia. Concomitant magnesium deficiency aggravates hypokalemia and renders it refractory to treatment by potassium.
Low magnesium leaves a potassium exit in the kidney stuck open, so potassium keeps leaking into the urine until magnesium is restored 4. That is why a doctor checks magnesium whenever potassium will not come up. As the magnesium deficiency page puts it, a confirmed low magnesium "often pulls potassium and calcium down with it", and that page lists diuretics and long-term acid-reducing PPIs among its common causes.
Low potassium with high blood pressure
Low potassium together with high or hard-to-control blood pressure is the classic pattern of primary aldosteronism, where the adrenal glands make too much aldosterone on their own. The Endocrine Society guideline recommends screening people with "hypertension and spontaneous or diuretic-induced hypokalemia" using the plasma aldosterone-to-renin ratio 5. A potassium that drops sharply on a modest diuretic dose fits the same pattern. The reverse does not hold: the high blood pressure page notes that primary aldosteronism sits behind "over 20% of hard-to-control cases, and most people who have it have a normal potassium".
The tests that find the reason
| Test | What it says about a low potassium |
|---|---|
| Magnesium | Low magnesium keeps potassium low until both are corrected 4 |
| Bicarbonate (CO2) | High points to vomiting, diuretics or aldosterone; low points to diarrhoea or renal tubular acidosis (a kidney condition that leaves the blood too acidic) |
| Urine potassium | A low result means the kidneys are holding on, which points to gut losses or low intake; a high result points to the kidney losing it 1 |
| Sodium, chloride, creatinine and eGFR (estimated kidney filtering rate) | Fluid balance and kidney function, which shape treatment. A low sodium alongside is common on thiazide diuretics. See kidney function explained |
| Glucose | High glucose and insulin both move potassium around |
| Aldosterone-to-renin ratio | Screens for primary aldosteronism when blood pressure is high 5 |
| ECG | Checks the heart when potassium is moderately or severely low, or there are symptoms 1 |
Tests a doctor often adds
How low potassium is treated
Treatment starts with the cause: a doctor may change a diuretic, stop a laxative habit, cut out licorice or treat the vomiting or diarrhoea. Mild lows are often handled with food plus prescribed potassium chloride tablets, and severe lows with IV potassium under heart monitoring 1. Because the blood shows only a sliver of the body's store, a small fall can hide a large deficit: each 0.3 mmol/L drop from losses reflects roughly 100 mmol missing 2.
Over-the-counter potassium supplements in the US are generally limited to 99 mg per serving, about a quarter of the potassium in a medium banana 6. Larger amounts come by prescription because too much potassium is as dangerous for the heart as too little. That risk is highest with chronic kidney disease or with blood pressure pills that hold potassium in: ACE inhibitors such as ramipril, ARBs such as losartan, and spironolactone.
Where a supplement has a role
Supports correction of a confirmed magnesium deficiency, under medical careMagnesiumLow magnesium can keep potassium low. When a blood test confirms low magnesium, doctors usually correct it alongside potassium [[4]]. Magnesium does not raise potassium when magnesium is already normal, and it is not a treatment for low potassium on its own; take it only as your doctor advises, especially with kidney disease.In our shop Foods high in potassium
The adequate daily intake is 3,400 mg for adult men and 2,600 mg for adult women, and most US adults eat less than that 6. Food supports recovery and keeps the level steady, but diet alone rarely fixes a true low caused by losses.
| Food | Portion | Potassium (approx. mg) |
|---|---|---|
| Dried apricots | 1/2 cup | 755 |
| Lentils, cooked | 1 cup | 731 |
| Potato, baked with skin | 1 medium | 610 |
| Kidney beans, canned | 1 cup | 607 |
| Yoghurt, plain nonfat | 1 cup | 579 |
| Banana | 1 medium | 422 |
| Spinach, raw | 2 cups | 334 |
| Salmon, cooked | 3 oz | 326 |
Figures are rounded from the NIH Office of Dietary Supplements potassium fact sheet 6. The wider picture of how potassium works with sodium, chloride and bicarbonate is in electrolytes explained and the electrolytes topic.
Read your own results
Read a low potassium beside magnesium, bicarbonate, sodium and kidney function rather than alone; the pattern usually names the cause. The potassium page covers the whole range, and the high potassium page covers the other direction.
Citations
- Kardalas E, et al. Hypokalemia: a clinical update. Endocrine Connections, 2018;7(4):R135-R146
- Gennari FJ. Hypokalemia. N Engl J Med, 1998;339:451-458
- Palmer BF, Clegg DJ. Physiology and pathophysiology of potassium homeostasis: Core Curriculum 2019. Am J Kidney Dis, 2019;74(5):682-695
- Huang CL, Kuo E. Mechanism of hypokalemia in magnesium deficiency. J Am Soc Nephrol, 2007;18(10):2649-2652
- Funder JW, et al. The management of primary aldosteronism: case detection, diagnosis, and treatment. An Endocrine Society clinical practice guideline. J Clin Endocrinol Metab, 2016;101(5):1889-1916
- NIH Office of Dietary Supplements. Potassium: Fact sheet for health professionals, 2022
- Cohn JN, et al. New guidelines for potassium replacement in clinical practice: a contemporary review by the National Council on Potassium in Clinical Practice. Arch Intern Med, 2000;160(16):2429-2436
Educational context only, not medical advice or a diagnosis. Always discuss your results with a clinician.
